This invention pertains generally to apparatus for effecting two-to-four wire conversion in communication systems. More particularly, the invention is directed to a subscriber line interface circuit (SLIC) providing electronic hybrid circuitry for interconnecting a two-wire subscriber loop to a four-wire transmission medium while simultaneously providing DC battery feed to the subscriber loop.
One typical prior SLIC is taught in U.S. Pat. No. 4,041,252 issued to Earl T. Cowden and assigned to the assignee of the present invention. That SLIC and variations of it have been widely and successfully utilized in the System 1210 electronic switching system manufactured and sold by International Telephone and Telegraph Corporation.
The Cowden SLIC includes provisions for battery boost, or range extension, applications such that the total DC battery potential injected into the line is increased. A combined battery and voice injection circuit is coupled to either side of the subscriber line by a resistance. For a line requiring a 900 ohm AC termination impedance, a 450 ohm resistor is connected to each side of the line.
A further consideration in the use of interface circuitry relates to the possible presence of lightning in the outside-plant circuitry. More specifically, lightning may strike tip and ring wires strung on poles with a resultant voltage surge traveling to the central office. While large voltage surges are limited by protection equipment at the central office, there is the possibility that a smaller voltage surge may travel beyond the protection circuitry and damage the sensitive electronics of the interface circuit. It thus becomes desirable to incorporate some form of protection into the design of the interface circuit, which protection would not interfere with normal operation of the interface circuit.